1. Field of the Invention
This invention generally relates to a digital communication control method, and, in particular, to a digital communication control method and system suitable for use with a digital facsimile machine.
2. Description of the Prior Art
Digital facsimile machines using a digital network as a transmission line are now in practical use. Such digital facsimile machines include those which may selectively use either digital circuit switching or packet switching networks as a transmission line. In digital facsmilie machines, data are typically transmitted as arranged in the form of a frame in accordance with a High-level Data Link Control (HDLC) procedure. The maximum size of data (user data) in such a frame is, typically, set by negotiation with a packet switching network to be connected in the case of using a packet switching network, and, alternatively, in accordance with data transmitting capabilities of both of a transmitter and a receiver in the case of using a digital circuit switching network. In addition, in order to obtain increased transmission efficiency, a predetermined coding process is customarily applied to data to be transmitted.
Typically, transmission of image information of one page in a digital facsimile machine is carried out according to the following procedure. After establishing a transmission line with a receiver, a transmitter determines a transmission function to be used and initiates reading of an original to be transmitted. When the amount of data thus read has reached a predetermined size, the data thus read is formed into a frame, which is then transmitted to the receiver. Upon completion of transmission of data of one page in this manner, the receiver responds by sending a predetermined signal to the transmitter.
Considering the cost of communication, the charge is calculated on the basis of the number of packets transmitted in the case of the packet switching network; whereas, the charge is calculated on the basis of the length of time during which a transmission line has been established in the digital circuit switching network. Thus, in the case of using a packet switching network, the cost of communication can be minimized if data is transmitted while maximizing the size of a frame to be used so as to minimize the number of packets. However, in the case of using a digital circuit switching network, if data is transmitted using a frame having a size which has been set, the cost of communication could increase undesirably under the following situation. That is, if an original has a portion which produces a number of all white lines, the amount of data to be transmitted for one page could become extremely small when compressed by coding. In such a case, the amount of data for one page may be less than the amount of data which can be transmitted by one frame. Even in such a case, a sequence of steps, including: reading of an original (by transmitter), compression by coding process (by transmitter), data transmission (by transmitter and receiver), decompression by decoding process (by receiver) and recording of received image information (by receiver) is carried out in sequence.
On the other hand, since the transmitter cannot disconnect the transmission line until it has received a response from the receiver, a sum of the processing time by the transmitter and the processing time by the receiver is reflected in the time period for keeping the transmission line open. For this reason, the communication time period tends to be longer, and, thus, the cost of communication tends to be higher.